A keyless entry system is one of a door unlocking system for a vehicle or buildings (for example, JP2003-138817A). In a conventional keyless entry system, an ID code which is transmitted from a portable device is received at a vehicle side and then collated with each ID code pre-registered in the vehicle. If both ID codes are coincident with each other, doors of the vehicle are unlocked.
According to this keyless entry system, a user presses down an unlocking switch provided on the portable device to transmit the ID code from the portable device to the vehicle. Alternatively, a request signal is transmitted from the vehicle side and then the portable device receiving the request signal transmits the ID code to the vehicle
This keyless entry system is very convenient because a mechanical key is not required to be inserted into a key cylinder in order to unlock doors of a vehicle. Particularly, in the case of the latter keyless entry system, it is unnecessary for a user to carry out a switch operation (press of an unlocking switch or the like). Thus the user can easily unlock the doors even when both hands of the user are unusable.
However, in the above keyless entry system, a dark current consumed in the vehicle increases because the request signal is intermittently transmitted, and thus the probability of battery run-down increases.
Furthermore, if the output level of the request signal is lowered to reduce the dark current, the communication with the portable device cannot be established because of noises or the like. It is thus impossible to acquire the ID code from the portable device. In addition, when the output interval of the request signal is lengthened to reduce the dark current, the system response associated with the unlocking operation is lowered.